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You may have seen the media images of bays and coastlines along Hurricane Irma’s track, in which the ocean has eerily “disappeared”, leaving locals amazed and wildlife stranded. What exactly was happening?

Most people are familiar with the idea that the sea is not at the same level everywhere at the same time. It is an uneven surface, pulled around by gravity, such as the tidal effects of the Moon and Sun. This is why we see tides rise and fall at any given location.

At the same time, Earth’s atmosphere has regions where the air pressure is higher or lower than average, in ever-shifting patterns as weather systems move around. Areas of high atmospheric pressure actually push down on the ocean surface, lowering sea level, while low pressure allows the sea to rise slightly.

This is known as the “inverse barometer effect”. Roughly speaking, a 1 hectopascal change in atmospheric pressure (the global average pressure is 1,010hPa) causes the sea level to move by 1cm.

When a low-pressure system forms over warm tropical oceans under the right conditions, it can intensify to become a tropical depression, then a tropical storm, and ultimately a tropical cyclone – known as a hurricane in the North Atlantic or a typhoon in the northwest Pacific.

As this process unfolds, the atmospheric pressure drops ever lower and wind strength increases, because the pressure difference with surrounding areas causes more air to flow towards the storm.

In the northern hemisphere tropical cyclones rotate anticlockwise and officially become hurricanes once they reach a maximum sustained wind speed of around 120km per hour. If sustained wind speeds reach 178km per hour the storm is classed as a major hurricane.

Surging waters

A “normal” storm surge happens when a tropical cyclone reaches shallow coastal waters. In places where the wind is blowing onshore, water is pushed up against the land. At the same time the cyclone’s incredibly low air pressure allows the water to rise higher than normal. On top of all this, the high waves whipped up by the wind mean that even more water inundates the coast.

The anticlockwise rotation of Atlantic hurricanes means that the storm’s northern side produces winds blowing from the east, and its southern side brings westerly winds. In the case of Hurricane Irma, which tracked almost directly up the Florida panhandle, this meant that as it approached, the east coast of the Florida peninsula experienced easterly onshore winds and suffered a storm surge that caused severe inundation and flooding in areas such as Miami.

The negative surge

In contrast, these same easterly winds had the opposite effect on Florida’s west coast (the Gulf Coast), where water was pushed offshore, leading to a negative storm surge. This was most pronounced in areas such as Fort Myers and Tampa Bay, which normally has a relatively low tide range of less than 1m.

The negative surge developed over a period of about 12 hours and resulted in a water level up to 1.5m below the predicted low tide level. Combined with the fact that the sea is shallow in these areas anyway, it looked as if the sea had simply disappeared.

As tropical cyclones rapidly lose energy when moving over land, the unusually low water level was expected to rapidly rise, which prompted authorities to issue a flash flood warning to alert onlookers to the potential danger. The negative surge was replaced by a storm surge of a similar magnitude within about 6 hours at Fort Myers and 12 hours later at Tampa Bay.

Rising waters are the deadliest aspect of hurricanes – even more than the ferocious winds. So while it may be tempting to explore the uncovered seabed, it’s certainly not wise to be there when the sea comes rushing back.

This is an article from I Have Always Wondered, a new series where readers send in questions they’d like an expert to answer. Send your question to alwayswondered@theconversation.edu.au

Why is the sea salty? – Robert Moran, Middlecove

The short answer is that water dissolves the salts contained in rocks, and these salts are carried in the water to the sea.

As raindrops form, they absorb carbon dioxide from the air. The water (H₂O) and carbon dioxide (CO₂) react to form carbonic acid (H₂CO₃). The carbonic acid makes rainwater slightly acidic, with a pH of around 5.6. Pure water has a pH of 7, which is neutral.

So, rain dissolves salts out of the rocks and these salts are carried via runoff to streams and rivers and finally to the sea. Rivers carry almost 4 billion tonnes of salt to the sea each year.

But rivers aren’t salty, right? Rivers are definitely not as salty as the sea, but they constantly carry their small salt content into the sea, and as a result the concentration of salt in the sea (which oceanographers call salinity) has built up over millions of years.

In fact, rivers aren’t the only source of sea salt. Rocks in the sea also play a role, and hydrothermal vents in the ocean floor and subsea volcanoes also supply dissolved salts to the sea.

Over millions of years, the concentration of salts has increased from possibly almost fresh in the primeval sea to where it is now – an average of 35 grams of salt in every kilogram of seawater.

If all this salt could be taken out of the ocean and spread over Earth’s land surface, according to the US National Oceanic and Atmospheric Administration, it would form a layer more than 150 metres thick.

Why are some places saltier than others?

Salinity varies from place to place in the sea, depending on how close you are to rivers, how much rain falls, how much evaporation occurs, and whether ocean currents are bringing in saltier or fresher water.

In general, the sea is saltier in the subtropics, where evaporation is high due to warm air temperatures, steady trade winds, and very low humidity related to atmospheric circulation patterns called Hadley Cells.

The sea is fresher close to the Equator where rainfall is high, and in the Southern Ocean and Arctic Ocean, where sea ice melt in the summer adds fresh water.

Enclosed seas, such as the Mediterranean and Red Seas, can be very salty indeed. This is because the removal of fresh water by evaporation is much larger than the addition by rainfall, and lower-salinity waters from the deep sea can’t flow in as easily.

Ocean salinity as a rain gauge

While the total amount of salt in the sea is pretty constant, the distribution of the salt is changing. Broadly speaking, the salty parts of the ocean are becoming saltier, and the fresh parts fresher.

These salinity changes are caused by changing rainfall and evaporation patterns globally, where wet places are generally becoming wetter and dry places are getting drier.

This amplification of the water cycle is a consequence of rising air temperatures due to climate change. Warm air can hold more moisture, so it can receive more evaporated water from the sea or land surface, and then release more when it rains.

Rainfall and evaporation are difficult to measure accurately, particularly over the ocean where 78% of rain falls.

Ocean salinity, on the other hand, is easier to measure now that we have the global Argo program: an armada of profiling floats that measure salinity and temperature from the surface to a depth of 2,000m, and surface salinity measurements via satellite.

Ocean salinity measurements are not only being used to understand past changes in the water cycle and reduce uncertainty in climate models, they are helping to improve seasonal rain forecasts around the world.

Despite appearances, the oceans are far from silent places. If you dunk your head underwater you’ll hear a cacophony of sounds from wildlife great and small, crashing waves, and even rain. And it’s louder still for creatures attuned to these sounds.

However, humans are changing these ocean soundscapes. Our recent research showed that changes caused by people, from ocean acidification to pollution, are silencing the seas’ natural noises. (We’re also filling the oceans with human noise).

This is bad news for the species that depend on these noises to find their way.

Ocean soundscapes

All over the world you can hear a lively crackling sound made by thousands of snapping shrimp that live along coastlines.

These common shrimp, often referred to as pistol shrimp, have a large claw that they can close with such force that a cavitation bubble is formed. As this bubble implodes on itself a loud snap is created – like a pistol shot – which can be heard over long distances.

In fact, snapping shrimp are the loudest marine invertebrates, and second only to the noisest marine animals, which are sperm whales! Snapping shrimp are found all over the world, including in coral reefs, kelp forests, seagrass beds and mangroves.

Other types of animals create ocean noise too. Urchins and parrotfish make clearly audible chomping sounds as they scrape algae off rocks. Many fish are frequent and loud talkers and make an array of sounds such as chirps, burps, whistles, knocks and so on. They use these to mark out their territory, during fights and to locate mates.

These biological sounds, together with those from rain, crashing waves and seismic activities, form the so-called underwater soundscape.

Learn more about marine soundscapes watching this video.

Sounds that are emitted from temperate and tropical reefs are loud and quite constant. As such these sounds form a reliable source of information for animals, particularly for navigation.

Most animals in the sea let go of their fertilised eggs without providing any parental care. As these eggs hatch, small babies (larvae) are dispersed by ocean currents. Growing up away from coastal areas provides a safer place with fewer predators.

However, after growing for a few weeks or months in the open ocean, it is time for these young animals to return to the coast to find a home. How do they find their way in the vast and uniform open ocean? Sounds and odours from coastal habitats are key cues that allow marine animals to find their new homes and replenish adult populations.

Going quiet

Humans are increasingly dominating the physical and chemical environment. We are altering the carbon cycle through the burning of fossil fuels and the nitrogen cycle by extracting vast amounts of nitrogen for food production and releasing it as waste. Large amounts of this carbon and nitrogen liberation end up in the ocean.

About one-third of the carbon dioxide that humans emit into the atmosphere dissolves in the ocean, leading to increased seawater acidity (or ocean acidification). This is an obvious problem for animals that produce a calcium carbonate shell or skeleton (such as corals, some plankton, and snails). Remarkably, ocean acidification also alters the behaviour of many animals by messing up their brain functioning.

Earlier studies (see also here) have shown that ocean acidification can change the response of fish larvae to settlement habitat sounds by deterring them rather than attracting them.

Learn more about the effects of ocean acidification on fish behaviour watching this animation video.

Two of our recent studies (see also here) showed that ocean acidification not only affects sound reception, but also the sounds that ocean ecosystems produce. If we don’t reduce greenhouse gas emissions, rocky reefs could be much quieter in 2100 than now. And snapping shrimps are the reason.

Coastal discharge of nutrients from sewage plants and catchment runoff also degrades kelp forests and seagrass beds. These coasts are more silent than their healthy counterparts.

In many parts of the world, kelp forests, seagrass beds and coral reefs have been replaced by carpets of turf-forming or mat-forming algae. These weedy types of algae have much lower diversity of species and provide less shelter and feeding opportunities for shrimps and other noisy animals.

Degraded habitat means fewer animals, which means less noise. For larvae that use sound as a navigational cue, this means that fewer larvae will be able to successfully locate their home. And fewer returning larvae means less replenishment of fish stocks.

The effects of ocean acidification on fish orientation and soundscapes.Dr Tullio Rossi

Options for restoration

Climate change and ocean acidification act at global scales and are difficult to stop in the short term. In contrast, nutrient pollution is a local stressor, which makes it more manageable.

Various options exist for local communities to reduce nutrient pollution of coastal areas. These include improved sewage treatment, restoration of coastal vegetation (such as mangroves) and swamps that extract sediment and nutrients from stormwater runoff, and decreasing the use of rivers as outlets for polluted waters.

Reducing the impacts of nutrient pollution on coastal ecosystems makes these systems more robust and provides them with increased resilience to cope with the impacts of ocean warming and acidification.

This summer has outstripped long-term sea surface temperature records that extend back to the 1950s. We have seen warm surface temperatures all around Australia and across most of the Pacific and Indian oceans, with particularly warm temperatures in the southeast and northern Australian regions.

In recent months, this warming has been boosted – just like land temperatures – by natural and human-caused climate factors.

Why so warm?

These record-breaking ocean temperatures around Australia are somewhat surprising. El Niño events, such as the one we’re currently experiencing, typically result in cooler than normal Australian waters during the second half of the year. So what is the cause?

The most likely culprit is a combination of local ocean and weather events, with a substantial contributor being human-caused climate change.

In the north, the recent weak monsoon season played a role in warming surface waters. Reduced cloud cover means more sunshine is able to pass through the atmosphere and heat the surface of the ocean. Trade winds that normally stir up the water and disperse the heat deeper into the ocean have also remained weak, leaving the warm water sitting at the surface.

In the south, the East Australian Current has extended further south over the summer. This warm current flows north to south down Australia’s east coast. Normally it takes a left turn and heads towards New Zealand, but this year it extended down to Tasmania, bringing warm waters to the south east.

This current is also getting stronger, transporting larger volumes of water southward over time. This is due to the southward movement of high pressure systems towards the pole.

High pressure systems are often associated with clear weather in Australia, and when they move south they prevent rain. This southward movement over time has also been linked to climate changes in our region, meaning that changes in both rainfall and ocean temperatures are responses to the same global factors.

We’ve also seen high ocean temperatures in the Indian Ocean. Around 2010, temperatures in the region suddenly jumped, likely because of the La Niña event in the Pacific Ocean. The strong events during this period transferred massive amounts of warmth from the Pacific Ocean into the Indian Ocean through the Indonesian region.

The warmer waters in the Indian Ocean have persisted since and have influenced land temperatures. The five years since the 2010 La Niña are the five hottest on record in southwest Western Australia (ranked 2011, 2015, 2014, 2013 and 2012).

What are the impacts?

The world’s oceans play a major role in global climate by absorbing surplus heat and energy. Oceans have absorbed 93% of the extra heat trapped by the Earth since 1970 as the greenhouse effect has increased. This has lowered the rate at which the atmosphere is warming – which is a good thing.

However, it also means the oceans are heating up, raising sea levels as well as leading to more indirect impacts, such as shifting rainfall patterns.

As a nation that likes to live by the coast as well as enjoy recreation activities and harvest produce from the sea, warmer-than-usual oceans can have significant impacts.

Australia derives a lot of its income from its oceans and while such impacts aren’t often seen immediately, they become apparent over time.

What is the outlook?

The seasonal outlook from the Bureau of Meteorology shows El Niño weakening over the next few months. This typically means cooler weather and can mean more rain on land.

However, closer inspection shows surface temperatures over the entire Indian Ocean and coastal Australian waters will very likely continue to remain well above average for the next few months. There are currently signs that surface currents are moving warm El Niño waters from the eastern Pacific over to the western Pacific, towards Australia.

There is potential for the East Australian Current to continue to transport this warmth to southern waters as far as Tasmania. Warm water could also be transported through Indonesia and travel south along the Western Australian coast via the warm Leeuwin Current, potentially causing further warming of already record warm waters.

So for the near future, the waters are going to continue to be warm. That’s good news if you’re heading to the beach, but not so good for the environment.

Australia’s reputation for deadly creatures of all kinds is known the world over. Tourists worry about it, and comedians have a field day with it. Here’s what Bill Bryson says in his book In a Sunburned Country:

[Australia] has more things that will kill you than anywhere else. Of the world’s ten most poisonous snakes, all are Australian. Five of its creatures – the funnel web spider, box jellyfish, blue-ringed octopus, paralysis tick and stonefish – are the most lethal of their type in the world.

Bryson certainly has a way with words. But, to be honest, he forgot a few things.

The long list

Australia has at least nine species of Irukandjis, a group of jellyfish so nasty that their drop-for-drop toxicity leaves the box jellyfish in the dust.

Impressive, considering the box jelly has long been considered the world’s most venomous animal. A massive sting from a box jelly kills in as little as two minutes; for other victims, it’s generally painful with some scarring, but that’s about it.

Irukandji, in contrast, with just an imperceptible brush of venom leaves almost no mark. But after about a half hour you develop Irukandji syndrome, a debilitating mix of nausea, vomiting, severe pain, difficulty breathing, drenching sweating and sense of impending doom. You get so sick that your biggest worry is that you’re not going to die!

And that’s just the beginning: up to a third of victims require life support and a quarter have ongoing complications, including permanent heart damage or neurological damage.

Bryson also forgot the blue bottles that sting some 25,000 to 45,000 people each year in Australia, at least one species of which causes Irukandji syndrome.

And he forgot the bullrout, which is kind of a brackish-water version of the stonefish – caution, they hang out at boat ramps and these suckers hurt.

And stingrays, which combine stabbing and venom into the one injury. And the cone snail, which looks mild-mannered, but can imperil your life with one stab of its lightning-fast barb.

Then there are sea urchins and stinging hydroids and venomous sponges, which will put you in a world of hurt. But nobody ever thinks to include them.

And the sea snakes: if you get one in your fishing net, or your dive equipment, or your hair, remember the old adage “don’t grab a snake by its tail”. Well, I’m not sure if that’s an adage or not, but it should be. In fact, “don’t grab a snake” would be better.

Bryson also forgot the world’s only venomous mammal, the platypus: males have a venomous spur on the back legs, and they seriously hurt. And my new favourite, the arrow worm. Yes, the arrow worm.

Granted, there aren’t any reported deaths from arrow worms, but they deserve respect. They look like a beansprout with fish fins, with a fish tail at one end and rows of big scary spines at the other, which they use to grasp their food. And they “bite” with tetrodotoxin – the same venom that makes fugu (the pufferfish delicacy) and blue ring octopus so lethal.

But why?

Okay, venomous beansprouts, swans and fear of not dying aside, what is it with Australia’s dangerous creatures? The typical explanation for powerful venoms is subduing dinner or dealing quickly with danger, especially for delicate creatures or those that aren’t able to track prey for long distances.

But certainly the box jellyfish’s venom is overkill, while the Irukandji takes too long. What’s more, fish don’t appear to get Irukandji syndrome … although I’ve never been sure how to tell if a fish is sweating.

Similarly, the dinner-or-danger hypothesis doesn’t seem to hold true for stabbing fish wounds, such as those delivered by stonefish, bullrouts and stingrays. Certainly, the stabbing must be far more effective than all but the most instant venom effects.

But one must keep in mind that these creatures evolved their toxins long before Homo sapiens fossicked the tide pools or snorkelled the reefs. So although their venoms can harm us, this may just be coincidental.

A question that often arises is what effect climate change will have on these creatures or their venoms. Well, the answer is we really don’t know yet.

With regard to species, there will be winners and losers. Many of the venomous sea creatures are tropical, and many tropical species are expanding southward. To what extent this may put the more populated southerly areas at higher risk is still unclear.

One group, however, seems particularly poised to benefit: the jellyfishes. As warmer water stimulates their metabolism, they grow faster, eat more, breed more and live longer. Irukandjis and box jellyfish become more toxic as they mature, so getting there faster and staying there longer could have undesirable outcomes for sea users.

How, then, can we possibly navigate these dangers when curious sea snakes want to swim with us, duckbilled platypus, stones and beansprouts must be viewed with suspicion, blue is sounding like the new warning colour, invisible jellyfish will lay us flat, and even the swans, a symbol of romance, are scary?

Four tips for keeping safe

Rule 1: First and foremost, try to make it a rule never to touch an animal that isn’t a personal friend. This will prevent the vast majority of bite and sting injuries, and not just from sea creatures.

Rule 2: Do the stingray shuffle when moving in sandy water: drag your feet in such a way that you’re continuously kicking sand in front to where you’re about to step. This will scare most creatures away so that you don’t step on them.

Rule 3: Wear protective clothing (a full-body lycra suit, for instance) when swimming in areas where box jellyfish or Irukandjis may appear. If stung by box jellyfish or Irukandjis or unknown jellyfish in the tropics, douse with vinegar to neutralise undischarged stinging cells.

The Australian government appears to be intent on scaling back wind farms in Australia. A Senate inquiry has recommended increasing regulation for wind farms in response to health concerns, and Prime Minister Tony Abbott recently commented to radio host Alan Jones that his government has managed to reduce the number of “these things” [wind turbines], but he personally would have preferred “to have reduced the number a whole lot more”.

But there’s another solution that would continue to build the capacity of wind energy while removing possible impacts on land-holders: put wind farms out to sea.

Terrible turbines?

The primary drivers for the government’s hostility to wind farms centre around the alleged socio-cultural and health impacts of wind turbines.

In the view of opponents, wind turbines represent an unsightly blight upon the landscape and cause intolerable noise pollution.

The Australian government’s stance is, however, increasingly out of step with the international community – both economically and morally.

In recent weeks the G7 group of nations announced their commitment to reduce emissions of carbon dioxide to 40-70% below current levels by 2050, and to eliminate the use of fossil fuels altogether by 2100. As a renewable energy source, wind farms can help to displace the use of fossil fuel generation in the electricity network.

And last week Pope Francis and his 183-page encyclical made a radical call to decarbonise and address climate change as a major existential issue.

The government-led attack on wind farms is therefore at odds with a global shift in the development of renewable energy with wind a leading technology in the renewable picture.

Stepping offshore

One approach that would serve to sidestep the problems of terrestrial wind farms (real or perceived) is to send the turbines offshore. Marine renewable energy, whether from wind, wave or tidal sources, is set to become a major supplier to global energy needs.

Among the differing technologies, offshore wind is emerging as the most efficient and competitive player with significant expansion in Europe and Asia.

In Europe more than 2,080 offshore turbines have been installed and connected to the grid in 11 European countries with a cumulative total of 6,562 megawatts (6.562 gigawatts, or GW) in 69 wind farms. Wind energy (both offshore and terrestrial) is a small but growing part of renewables produciton in the EU, consisting of 10.5 % of the EU-28’s renewable energy produced in 2013. The United Kingdom is the leading producer of offshore wind energy, with installed capacity of 4.5 GW, a further 12.6 GW in construction or approval, and 5.2 GW in planning.

Ambitious future forecasts include 40 GW of European offshore wind by 2020, meeting 4% of the EU’s electricity demand with a further 110GW to be installed between 2020 and 2030 that would meet 14% of EU demand.

In China 0.67 GW of offshore wind capacity is installed, with more on the way as renewable energy is increasingly recognised as an important and growing element of China’s energy mix. China produced 450 GW of renewable energy in 2014.

Meanwhile South Korea is forecast to become a major strategic player with numerous offshore wind farm sites in the planning phase, investing US$9 billion into a massive 2.5 GW wind power development led by Korea Electric, one of the world’s leaders in tidal energy production.

Small-scale but growing

In terms of the overall energy picture the amount of installed capacity from marine sources – wind, wave and tidal – is presently small. Of the 369 GW of global wind production, only 8.7 GW (2.3%) is from offshore wind.

Total ocean renewable energy as a proportion of the global renewable capacity (which includes hydropower and onshore wind) is also tiny, currently just 0.5%.

This situation is expected to substantially shift in coming years as terrestrial systems reach capacity in terms of competition for space, social opposition or in limits to generative capacity and the concept of the blue economy gains momentum in coastal states and regional clusters such as the European Union and the Asia-Pacific Economic Cooperation (APEC) countries.

Opportunities and challenges

Australia has a long coastline and abundant offshore wind, wave and tidal energy resources at its disposal. Pushing wind farms offshore would seem to circumvent the main objections to wind turbines on land whilst enabling the renewable energy sector an opportunity to grow.

Providing such an avenue for the renewable energy sector would grow the innovation and manufacturing base providing an avenue for a high skilled and technical workforce and giving Australia a stake in a growing global market.

Emerging technologies such as wave and tidal, while presently small, have been recently supported by coalition ministers including a recent world-first connection of a wave hub in Western Australia.

Arguably moving wind turbines offshore merely transfers the burden of their visual impacts to sea. Crucially though, few people (indeed, vocal voters) live where such turbines might be located.

Offshore wind turbines represent proven technology but are likely to be more expensive both to build and maintain, but recent estimates highlight that costs are falling, and are potentially cheaper than gas fired or nuclear options.

It can also be anticipated that existing marine users such as fishermen, are unlikely to welcome such a “new” and potentially competing offshore activity. That said, there are ways and means to overcome such apparently conflicting uses, for instance through processes of marine spatial planning that are emerging worldwide.

While implementation challenges exist for offshore wind energy developments, this option offers a pathway for Australia to stay in the renewable energy game, reduce our carbon emissions and develop innovative new industries.